The blood that flows through the brain distributes nutrients to the brain and removes wastes. This flow maintains the high rate of metabolism necessary for the brain to function. Restrictions in blood flow may occur from vessel narrowing (stenosis), clot formation (thrombosis), blockage (embolism), or blood vessel rupture (hemorrhage). Lack of sufficient blood flow (ischemia) threatens brain tissue and may cause a stroke.
The flow of blood through the arteries in the brain can be analyzed using transcranial Doppler ultrasonography (TCD). Blood flow velocity is recorded by emitting an acoustic wave from the ultrasound probe, which then is reflected by various materials. The TCD probe measures the reflected acoustic wave. The direction and the speed of blood flow relative to the TCD probe can be measured by determining the phase shift of acoustic wave reflected from objects in the blood, such as red blood cells. For example, red blood cells moving away from the probe cause the frequency to decrease and faster moving red blood cells cause a greater change in the frequency. Combined with other tests, this information can be used to locate restrictions in the blood vessels in the brain, and to track changes in blood flow over time. In this way, TCD gives valuable information about the site of a stroke and the patient's progress after a stroke. TCD is also used to evaluate the contraction of blood vessels that can occur if a blood vessel ruptures.